thread 'rustc' panicked at 'found unstable fingerprints for optimized_mir #85064

First of all, thanks for all your hard work on the compiler!

I ran into the following issue while doing a cargo build of the following repo: parity-bridges-common. I'd been check-ing and build-ing my code fine but at one point just got stuck in this state where it would panic with this message on every build.

I wasn't able to figure out how to produce a minimal example. I also wasn't able to reproduce it by just building the offending packages (e.g millau-bridge-node or rialto-bridge-node based off the error output) individually.

Unfortunately (or maybe fortunately?) I wasn't able to reproduce by building with the nightly compiler.

Meta

rustc --version --verbose:

$ rustc --version --verbose
rustc 1.52.0 (88f19c6da 2021-05-03)
binary: rustc
commit-hash: 88f19c6dab716c6281af7602e30f413e809c5974
commit-date: 2021-05-03
host: x86_64-unknown-linux-gnu
release: 1.52.0
LLVM version: 12.0.0

$ rustc +nightly --version --verbose
rustc 1.54.0-nightly (bacf770f2 2021-05-05)
binary: rustc
commit-hash: bacf770f2983a52f31e3537db5f0fe1ef2eaa874
commit-date: 2021-05-05
host: x86_64-unknown-linux-gnu
release: 1.54.0-nightly
LLVM version: 12.0.0

Error output

I captured the following error output after running RUST_BACKTRACE=1 cargo build. Since the output is so large I thought it would be best to put it in a Gist.